Fungal diseases are responsible for damage to many cultivated species. The amount of damage varies each year, depending on temperature, amount of rain, and quantity of inoculum present in fields. In some instances, fungal diseases can completely destroy fields, leading to an estimated average loss of yield of 20% on crops worldwide. Blackleg, one of the predominant fungal diseases in rape plants, typically results in losses of tens of millions of dollars annually. Sclerotinia, another predominant fungal disease of Cruciferae plants, which includes Brassica plants, as well as 400 other species of plants, including Compositae plants such as sunflower and leguminous plants such as pea, also can result in significant economic losses.
Blackleg disease is caused by an Ascomycetes fungus whose perfect or sexual form is Leptosphaeria maculans and whose imperfect or asexual form is Phoma lingam. The sexual form provides the primary inoculum each year and is responsible for the high variability of the fungus. L. maculans is in fact a complex of species, of which two main groups have been identified, TOX+ and TOX°. The TOX+ species is aggressive and produces two toxins, sirodesmin and phomalide. Within the TOX+ species, several strains or pathogenicity groups (PG) exist. In Europe, Australia, and Eastern Canada, PG3 and PG4 currently are the predominant strains of Phoma, while in Western Canada, PG2 currently is the predominate strain of Phoma. Damage to rape plants in Europe is typically limited to attack of the crown, but in North America, necrosis can occur both on the crown and the stem.
White rot or sclerotinia disease also is caused by an Ascomycetes fungus, Sclerotinia sclerotiorum. S. sclerotiorum is the sexual form of the fungus and provides the primary inoculum each year.
Strategies for limiting fungal damage include prophylactic measures such as crop rotations or burying of crop debris, fungicide use, and genetic improvement. Prophylactic measures, however, are not very effective as the fungus can survive for many years in the soil. Fungicides can be effective when applied at the appropriate time, but cost often is high compared with any gain in yield. Furthermore, genetic improvement in plants for resistance to S. sclerotiorum has been limited as only low tolerance to the fungus exists in various plant species. Varieties of rape plants that are tolerant to the PG3 strain of L. maculans, which contain a single resistance gene, have lost efficacy due to adaptation of the fungus or dilution of the tolerance factors in new rape plant genotypes. Thus, a need exists for plants that are resistant to such fungi as well as for methods of improving plant resistance to fungi.